This invention relates to fluid polymer suspensions that contain water, a dissolved salt, and a water-soluble polymer in a suspended form. In particular, this invention relates to suspensions of methylcellulose (MC), methylhydroxypropylcellulose (MHPC) and methylhydroxyethylcellulose (MHEC) in aqueous salt solutions.
Prior to the present invention, methylcellulose derivatives such as MC, MHPC, and MHEC have traditionally been handled in their dry, particulate form. Problems associated with dry methylcellulose derivative polymers include undesirable dust generation, poor dispersibility when added to aqueous systems, and undesirably long dissolution times.
The dust associated with dry, particulate methylcellulose derivatives present the same conventional handling problems as are encountered with other particulate water soluble polymers. When added to an aqueous system, dry methylcellulose derivatives tend to agglomerate to form clumps. Agglomeration can be reduced in many cases by adding the polymer to the aqueous system slowly with agitation. Slow dissolution, however, substantially reduces the speed of manufacturing operations.
Several formulations exist in the prior art that describe suspension systems of various water soluble polymers that avoid the above mentioned problems. For example, U.S. Pat. No. 4,283,229 (Girg et al.) discloses that stable suspensions of nonionic cellulose ethers can be prepared in a solution of 4 to 12% electrolyte if alumina is added to the suspension. Prior art aqueous fluid suspensions of water soluble polymers are further described in U.S. Pat. Nos. 4,883,536 and 4,883,537. U.S. Pat. No. 4,883,536 discloses the use of ammonium salts such as diammonium sulfate for preparing fluid suspensions of water soluble polymers. U.S. Pat. No. 4,883,537 discloses the use of concentrated aqueous potassium carbonate for preparing suspensions of sodium carboxymethylcellulose.
The present invention is directed to an aqueous suspension comprising at least 20% by total weight of the suspension, of methylcellulose derivatives having a bulk density of 0.3 g/ml or greater dispersed in an aqueous solution of at least one salt dissolved therein.
This invention also relates to a method of preparing a fluidized suspension of methylcellulose derivatives in aqueous salt solutions with a solids content of 20% by weight or greater of the suspended water-soluble polymer by using a methylcellulose derivative polymer having a bulk density of 0.3 g/ml or greater.
It has been discovered that the proportion of water soluble methylcellulose derivatives that can be suspended in any given aqueous salt solution is dependent upon the bulk density of the polymer in its dry pre-suspension state. It was found that samples of methylcellulose derivatives with a bulk density of greater than 0.30 could be suspended in aqueous salt solutions at a polymer content of 20% by weight or higher to form fluid pourable suspensions. Conversely, samples with a bulk density of less than 0.30 formed thick pastes when added to aqueous salt solutions at a water soluble polymer content of 20% by weight or greater.
The higher bulk density methylcellulose derivatives are therefore much more useful than the lower bulk density polymers for preparing fluid polymer suspensions in aqueous media since it is desired to have as high a water soluble polymer solids content as possible, while still allowing the fluid to be pourable. The higher the bulk density of the methylcellulose derivative, the better are the suspension properties. It is also desirable to prepare fluid polymer suspensions (FPS) with as high as possible a ratio of the water soluble polymer to the salt in the fluid suspension. A high water soluble polymer/salt ratio in fluid polymer suspension was found to translate into better performance properties in industrial applications to which these products are added compared to fluid polymer suspensions in which the ratio of the suspended water soluble polymer to FPS salt content was lower.
Generally, a large number of salts (i.e., organic or inorganic) can be used in this invention as long as the salt is at least soluble to the extent of 4 grams per 100 cubic centimeters as shown in the Handbook of Chemistry and Physics, published by CRC Press, Inc., 55th edition, pages B-63 through B-156. In practice, salts or combinations of salts of di- or tri-basic acids are used at lower levels (i.e., 4-15 weight percent of slurry system) while salts or combination of monobasic acids are used at higher levels (i.e., 20 weight percent or greater of slurry system) to effect preparation of a stable methylcellulose derivative suspension.
The present invention, fluid polymer suspensions of high bulk density methylcellulose derivatives in aqueous salt solution, has industrial applicability for uses such as for joint compound manufacture, tile cement thickening, food additives, adhesives and in other water based systems.
The variation in bulk densities among different samples of methylcellulose derivatives can often be explained by the processes employed to prepare these polymers. A wide variety of methylcellulose derivatives of varying bulk densities is generally available in the marketplace.
1. xe2x80x9cPourabilityxe2x80x9d of a Fluidized Polymer Suspension, as used in the following Examples, is determined by placing 100 grams of the suspension in question into a 4 ounce wide mouth jar, shaking well for 60 seconds, and then inverting the jar so that the opening of the jar protrudes above a catch vessel. A xe2x80x9cpourablexe2x80x9d suspension is one that discharges at least 20% of the weight of the suspension from its original container into the receiving vessel in a period of less than 60 seconds.
Using the test procedure for pourability, it was corroborated that the suspensions named as being xe2x80x9cfluid pourable suspensionsxe2x80x9d in the examples below, were xe2x80x9cpourablexe2x80x9d under the conditions cited above while those polymer suspensions that were described as xe2x80x9cthick pastesxe2x80x9d were found to be xe2x80x9cnot pourablexe2x80x9d in that they did not discharge any of their starting weight into the receiving vessel utilizing the pourability test method.
2. Bulk Density of Methylcellulose Derivatives
A weighed sample is placed in a graduated cylinder and tamped until the material reaches a constant volume. This volume is read and density calculated.
Apparatus
1. Laboratory Bulk Density Tester.
Procedure
1. Thoroughly mix the sample to assure uniformity.
2. Weigh out a measured quantity of sample and pour into a clean, dry graduated cylinder.
3. The cylinder should be tapped onto a hand surface for 20 minutes to allow settling of the sample in the tester, set the time for 20 minutes and turn on.
4. When the 20 minutes have been completed, position the graduaated cylinder to obtain a level reading. Read the volume occupied in milliliters.
Calculation
1. Divide the number of grams weighed (50 g) by the volume occupied in ml; this gives Bulk Density in g/ml.
2. Report the result to the nearest 0.01 unit.
3. Alternatively, use Bulk Density as reported by manufacturer for particular methylcellulose derivative.